How can I map two dissimilar objects to the same UV border?
Nathan, in fact it's more often the other way, meaning if you have two UV islands of the same model that share the same texture space it's much harder to avoid visible seams, even if those UV islands are exact copies of each other. It's quite impossible to have no visible seams at all if the two UV islands are only somewhat similar. 

The next big disadvantage of sharing texture space is that it's nearly impossible to paint away the seams, since you'd paint both parts instead of one.